The development of conventional laser induced techniques to detect laser induced effects on integrated circuits have resulted in scanning microscopes becoming useful tools for active fault localization in the area of integrated circuit testing. These conventional laser induced techniques generally involve using a scanned laser beam, typically in the infrared frequency range, to stimulate integrated circuit failures which are sensitive to thermal or carrier stimulations. These techniques have been found to be effective in localizing short circuits or open circuits in the metallization layers and the active regions of integrated circuits. Examples of these techniques include Optical Beam Induced Resistance Change (OBIRCH), Thermal Induced Voltage Alteration (TIVA), Thermal Beam Induced Phenomenon (TBIP), Externally Induced Voltage Alteration (XIVA) and Differential Resistance Measurement (DReM).
However, with the advancement of integrated circuit technology which has typically involved the use of more metallization layers and new low k inter-layer dielectric materials with lower thermal conductivity, the laser coupling efficiency is reduced. As a result, the detection sensitivity of these conventional laser induced techniques is also reduced. Accordingly, in order for these conventional laser induced techniques to remain effective, especially when used for the more advanced integrated circuits, an improvement in their detection sensitivity is needed.
A conventional approach to improve the detection sensitivity of these conventional laser induced techniques is to increase the power of the laser beam used, in order to compensate for the reduced laser coupling efficiency. However, there is a limit on the maximum power of the laser beam which can be used, since there may be potential laser induced damage on the integrated circuit under test when the power of the laser beam used is too high. Accordingly, the improvement in detection sensitivity from using this approach is obtained at the cost of a higher risk of damage to the integrated circuit under test. Therefore, this approach may not be desirable.
Another conventional approach to improve the detection sensitivity of these conventional laser induced techniques is to use a pulsed laser in conjunction with a lock-in amplifier. It has been found that the detection sensitivity is increased by pulsing the laser beam at suitable frequencies, where the noise level is low, in conjunction with the use of the lock-in amplifier for subsequent signal processing.
In order to achieve the desired level of detection sensitivity, it is required in this approach to have accurate calibration and fine control of the lock-in amplifier parameters, such as the time constant, the lock-in frequency and the phase difference between the reference frequency and the frequency of pulsing the laser beam for each scanning speed used. However, accurate calibration and fine control of the lock-in amplifier parameters is typically difficult to achieve in practice. In view of this, this approach is difficult to implement, and therefore, is not used in a real-time integrated circuit testing environment.